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LLC Resonant Half Bridge Converter
Asia Tech-DayAugust 17 to 27, 2009
Hong HuangApplications Engineer
Outline• Introduction to LLC resonant half bridge converter
– Benefits– Operation principle– Design challenges
• Design method– Transformer turns ratio selection– Magnetizing inductor selection– Resonant component selection
• Other design issues for LLC resonant converter– Current limiting– Soft start– OVP and Burst Operation
Design Challenges for DC/DC• Higher power conversion performance
– Higher efficiency, smaller heat sink– Higher switching frequency, smaller magnetics– Less energy storage capacitors, smaller size (e.g., for PFC holdup)– Moderate frequency variations
• Wide input voltage variations– AC-DC applications: holdup time requirement (PFC from 400V to
300V during holdup)• Larger Energy Storage Capacitor – high cost, large size, more
space• Converter ability to tolerate the variations
– DC-DC applications• Telecom, 36 to 75V (32V to 78V)• Some applications even asking 4:1 variations
• Wide output voltage trimming
Benefits of LLC Resonant Converter
• ZVS can be achieved by utilizing transformer magnetizing inductor
• Capacitor filter, less voltage stress on rectifiers
• Smaller switching loss due to small turn off current
• Variable switching frequency control, not sensitive to load change
• Frequency variations can be designed narrower compared to SRC
• Wide operation range without reducing normal operation efficiency
LLC Resonant Converter with Wide Operation Range
• fsw is set at resonant frequency at nominal input and output
• fsw is adjusted by feedback loop at other operation conditions
o
in
VVn 2/
=Transformer turns-ratiorrCL
fπ2
10 =Resonant
frequency
Square Wave Generator to excite Resonant Circuit
Rectifier to get DC
Operation PrinciplesAt Resonant Frequency
Vp
• Q2 and D2 ON, Q1 and D1 OFF• Magnetizing current in Lm, im• Cr resonates with Lr, ir• Cr and Lr deliver energy to output
Vg_Q1
Vg_Q2
Vp
ir
im
is
t
t
t
t
t
t0 t1 t2 t3 t4
Vg_Q1
Vg_Q2
Vp
ir
im
is
t
t
t
t
t
t0 t1 t2 t3 t4
Operation PrincipleAbove Resonant Frequency
• When switching frequency is above resonant frequency, circuit behaves as SRC
• Secondary current becomes CCM, reverse recovery loss increases
Vp
t0 t1 t2 t3
Vg_Q1Vg_Q2
Vp
ir
im
is
Operation PrincipleBelow Resonant Frequency
• When switching frequency is below resonant frequency, magnetizing inductor begins to participate in resonant and increases voltage gain
• Secondary diode becomes discontinuous
Vp
LLC Resonant Converter Gain Function
( )ennnnn
nn
QLjfffLfLM
+−+=
1)1( 22
2o
rmsosoe nVVV
π22
1, ==
nIII ormsos /)
22(1,Reπ
==
LLC Resonant Converter with Wide Operation Range• Unity gain is reached at Vr=(VLr-VCr)=0, where input voltage in phase with output voltage, and input voltage applies to load (Re) directly
Inductive Region, Ir lagging Vge
Capacitive Region, Ir leading Vge
Unity Gain, Vr=0Voe in phase with Vge
LLC Resonant Converter with Wide Operation Range
Working regions (Modes)- Inductive Region, if resonant network current is lagging input voltage- Capacitive Region, if resonant network current is leading input voltage- Resistive Region, if resonant network current is in phase with input voltage (boundary to divide Inductive and capacitive, by let the imaginary part zero of the input impedance)- Unity gain happens at (VLr-VCr)=0, where input voltage in phase with output voltage, and input voltage applies to load (Re) directly
LLC Resonant Converter with Wide Operation Range
• Should operate in ZVS region (Inductive Region, Ir lagging Vge)
• Avoid ZCS region (Capacitive Region, Ir leading Vge)– Hard switching of half bridge switches– Reverse recovery losses in primary FET body diodes– Large spikes on switch node– Higher EMI levels– Frequency relationship reversed
• Frequency increases as load increases
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0.1 1 10fn, (Ln =1)
Gai
n
Qe=0.0
Qe=0.2
Qe=0.8
Qe=1
Qe=2
Qe=10
ZCS/ZVS
Gain Characteristics with Ln and Qe
( ) rmrp
CLLf
+=
π21
0fffn =
Le RnR 22
8π
=e
rre R
CLQ
/=
Qe is related to output load
Qe Increasing with Load Current
rrCLf
π21
0 =
Qe increase with Ln constant (designed Lr, Cr, and operational RL),- Peak-gain becomes lower- Frequency at peak-gain moving to right- Better “frequency-selective
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0.1 1 10fn, (Ln =1)
Gai
n
Qe=0.1Qe=0.2Qe=0.5Qe=0.8Qe=1Qe=2Qe=5Qe=8Qe=10ZCS/ZVS
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0.1 1 10fn, (Ln =5)
Gai
n
Qe=0.1Qe=0.2Qe=0.5Qe=0.8Qe=1Qe=2Qe=5Qe=8Qe=10ZCS/ZVS
Impacts of Circuit Parameters
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0.1 1 10
fn
Gai
n
Qe=0.1Qe=0.2Qe=0.5Qe=0.8Qe=1Qe=2Qe=5Qe=8Qe=10ZCS/ZVS
Ln=1Ln=10Ln=5
Gain Change with Ln and Qe- Ln increase with Qe constant (designed Lr, Cr, and operational RL),
- flat, - magnitude shift-up, - frequency value at peak-gain moving towards more left,- less “frequency-selective”- wide frequency variation from no load to full load (more discussion later)
Ln somewhere 3 to 5 look best balance between peak gain and frequency change – can be initially pick-up
LLC Resonant Converter Operation for design consideration
• Operation/design with no load and minimum gain at a1• Operation /design with full load and maximum gain at a3• All gain curves cross at unity at fn=1, or f=f0• Qe design consideration at Heavy load, OCP, Short Circuit
Design Goals for LLC Resonant Converter
• Minimize RMS current under normal operation condition• Ensure ZVS operation• Ensure desired operation range
22
2Re
2,
2222
⎟⎟⎠
⎞⎜⎜⎝
⎛+
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛→
=+==
nI
LnVFHA
IIII
o
m
O
mrRMSP
πωπ
Design Consideration -1: Primary RMS Current at Normal Operation
• Primary current can be easily calculated
from the phasor circuit
• Primary side RMS current is summation
of magnetizing current and load current
• Larger Lm is better for less conduction
losses
Design Consideration -2: Secondary winding RMS Current
• Secondary side current is the difference between resonant tank current and magnetizing current
bridgeIIInII
tappedcenterIIInII
OSpeakoSRMS
OSpeakoSRMS
,222
,424
2/)(
_Re_
_Re_
ππ
ππ
=⇒=×=
−=⇒=×=
Design Consideration -3: Zero Voltage Switching
4,T
LnVI
m
opeakm =
Im,peak
ineqdeadpeakm VCtI 2, ≥×
22, )2(
21)(
21
ineqpeakmrm VCILL ≥+
ZVS conditions:- Enough H-field energy to balance E-field Energy in less than half cycle - Enough time to make the energy conversion- Worst operation for ZVS,
-Vo,min, Im,peak becomes small-Vin,max, more Ceq energy needs to discharge
eq
deadm C
tTL
16×
≤
Trade-off Design of Dead Time
mL
Smaller turn off currentSmaller magnetizing current
deadt
Increase RMS current due to duty cycle loss
Larger magnetizing currentLarger turn off loss
mL deadt
Trade-off Design of Dead Time
Trade-off between the switching loss and conduction loss on a case of 100ns dead time
Switching lossSwitching lossConduction lossConduction loss
Loss
(%)
Total lossLo
ss (%
)Total loss
Peak Gains with Ln and Qe
Initially select Ln in the range of 3 to 5 (gain curve not very flat and able to narrow down the frequency change while there is still enough gain)
Find proper Qe to get enough peak gain
Design Flow Chart for LLC Resonant Converter
out
oLe P
VnRnR 2
222
288ππ
==
Converter Specifications
choose an Ln and Qe
O
in
VVn2
=
Check max gainagainst graph
Peak gain enough? Yes Calculate Re
No
Change Ln and Qe
Resonant Capacitor
rnm LLL =
Resonant Inductor
( ) rswr Cf
L 221
π=
sweer fRQ
Cπ2
1=
Magnetizing Inductance
Design - Over Current Protection
During over load condition, check if the converter enters ZCS region
ee R
CrLrQ =
Max load
Overload?Required
Gain
Design - Soft start
Soft start is achieved by frequency control
Design – OVP and Burst Operation
• Burst Operation• To cover no load operation, smaller Ln is needed which increases
circulating (magnetizing) current, leading more conduction losses.• To reduce switching losses, ZVS is still needed at no load. This requires
higher magnetizing current, too.• To maintain output regulation, burst operation at light load and no load is an
alternative to balance switching losses and conduction losses.
• Secondary OVP• Feedback loop fault may cause
output over voltage.• Slow loop response may also
cause OVP.• Independent OVP circuit is
needed.
Feedback Loop Design
- Measure Gm(jω)- Design Gc(jω) based on Gm(jω) measurement
10 100 1 .103 1 .104 1 .105 1 .10660
40
20
0
20
40
60
180
120
60
0
60
120
180Gain of Type I Compensator
Gai
n (d
B)
60
60−
Gc f( )
180
180−
θc f( )
10610 f
)12
(
121)(
_+
××
+×=
optop
optodcc
fSk
S
RCS
KSG
π
UCC25600 EVM Modulator Plot (Test)
-60
-40
-20
0
20
40
60
10 100 1000 10000 100000 1000000
Frequency (Hz)
Gai
n (d
B)
-180
-120
-60
0
60
120
180
Phas
e (D
eg)
Summary
• Due to low switching losses, LLC resonant converter is able to operate at high switching frequencies, while maintaining high efficiency
• LLC resonant converter design needs to find a suitable magnetizing inductor to ensure small conduction losses and switching losses
• LLC resonant converter is able to achieve wide operation together with high efficiency
• By choosing a suitable Ln and Qe value, desired voltage gain can be achieved to input and output voltage variation range
• Complete system features– Programmable soft start– Programmable dead time– Programmable maximum/minimum switching frequency– 0.4A source, 0.8A sink driving capability– Simple ON/OFF control– Burst operation at light load condition
• Precise timing control– 3% accuracy on minimum switching frequency setting with only external
resistor– ±50ns matching on dead time– Soft start timer range from 1ms to 500ms
• Complete protection functions– Two levels over current protection, auto recovery and latch off– Bias voltage UVLO and OV protection– Over temperature protection– Soft start enabled after all fault conditions
• 8 pin SOIC package, simplifies design and layout
UCC25600 Resonant Half Bridge Controller
Programmable dead timeFrequency control with minimum/maximum frequency limitingProgrammable soft start with on/off controlTwo level over current protection, auto-recovery and latch upMatching output with 50ns tolerance
Application Circuit
UCC25600
Test based on EVM (UCC25600EVM)
Test with EVM: Resonant Tank
Q2 Vds
Ch3: Lr CurrentScale: 3.4A/div
Primary Peak Current: 2.72ASecondary Peak Current (Nt=16.7):
3.4A/div x 0.8div x 16.7 = 45.4A Load Current: 25A
Io/Is = 25/45.4 =0.55Compare: half wave rectifier
TP2 TP13
TP6
TP24
Test with EVM: Ripple and Hiccup
Test with EVM: Freq and Feedback
Frequency Variation Feedback Loop Bode Plots
Test with EVM: Efficiency and Load Regulation
Efficiency Load Regulation
Thank You!